Writing/modifying of epubs

[ghost]

I was thinking about writing a small library for editing the opf file in epubs but felt like it should instead be a part of this crate which I use for reading the metadata anyways. It would be nice if this library had APIs for not only reading but writing epubs, although that may be out of scope for this project.

[danigm]

I think it's a good idea to add an API to edit files.

To do that we should implement the save method, to pass from memory to an epub file, because right now there's only code to read a file and store it in memory as a tree with custom structs

[wingertge]

I'd also add constructing Epub files from in-memory data to that, for things like conversion between formats. I'd be willing to implement these changes myself and make a PR.